1. Field of the Invention
The present invention relates to an aqueous absorptive polymer-containing resin composition-producing method, an aqueous absorptive polymer-containing resin composition, and a porous substance-producing method using the aqueous absorptive polymer-containing resin composition and a porous substance, an insulated electric cable-producing method, an insulated electric cable and a coaxial cable.
2. Description of the Related Art
In recent years, with continuing miniaturization and high-density packaging of medical precision electronic equipment and communications equipment, electric wires/cables to be used therein are also increasingly designed to be thinned in diameter. Further, for signal lines and the like, there is a notable tendency to demand higher-speed transmission signals, and it is therefore desired that an insulator layer for electric wires to be used in the signal lines is thin and has as low a dielectric constant as possible so that transmission signals are thereby designed to be high-speed.
This insulator conventionally uses a foamed low-dielectric constant insulating material such as polyethylene, fluororesin, or the like. As the foamed insulator layer formation, there are known methods by winding or extruding a foamed film around a conductor, especially the method by extrusion is widely used.
The foaming method is broadly divided into physically- and chemically-foaming methods.
As the physically foaming method, there are a method by injecting into a melted resin a volatile foaming liquid such as liquid CFC to thereby cause its evaporating pressure for foaming, a method by injecting a foaming gas, such as nitrogen gas, carbon dioxide, or the like, directly into a melted resin in an extruder, to thereby produce uniformly distributed cellular micro closed cell foams in the resin, and so on.
As the chemically foaming method, it is well-known that by dispersing and mixing a foaming agent in a resin, the resin is molded and subsequently heated to thereby decompose the foaming agent to produce foaming gas.
For the physically foaming method, refer to JP-A-2003-26846, and for the chemically foaming method, refer to JP-A-11-176262.
However, the method by injecting into a melted resin a volatile foaming liquid has the limits of molding thin because of strong evaporating pressure, and difficulty in micro bubble formation or homogenous formation. Also, there is the problem of slow speed of injecting a volatile foaming liquid, therefore difficulty in high-speed producing, and poor productivity. Further, the method by injecting a foaming gas directly into a melted resin in an extruder has the limits of extruding thin diameter and thinning, and the problem of special facilities or techniques required for safety, therefore poor productivity or high producing cost.
On the other hand, the chemically foaming method has the problem that because by kneading, dispersing and mixing a foaming agent beforehand, the resin is molded and subsequently heated to thereby decompose the foaming agent to produce foaming gas, the resin molding temperature has to be held at a lower temperature than the foaming agent decomposition temperature. Further, there is another problem that a thin wire diameter tends to cause the wire to be broken by resin pressure in covering by extrusion, and therefore makes high-speed signal transmission difficult.
Also, the physical foaming using CFC, butane, carbon dioxide, or the like has the problem of large environmental loads, and the chemically foaming has the problem of high prices of foaming agents to be used.